The present invention generally relates to a surface acoustic wave device (hereinafter referred to as SAW device in abbreviation). More particularly, the invention concerns a SAW device capable of handling electric signals of high power and/or a large amplitude.
At present, the SAW device finds a wide range of utilization and application and is employed as a SAW filter adapted for transmission of high electric power or as a SAW resonator in which surface wave of a large amplitude is continuously present as a standing wave. However, the SAW devices mentioned above suffer shortcomings in that defects similar to hillocks, voids or the like due to electro-migration which take place in Al-metallized electrodes in semiconductor IC devices are produced in fine fingers of aluminium (Al) serving as SAW transmission/reception electrodes and reflector electrodes in the SAW devices, as is reported in "Thin Solid Films", 64, 9-15 (1979) (J. I. Latham et al), pp. 9-14 and "Transactions of The Institute Of Electronics Communication Engineers of Japan", Vol. J67-C, pp. 278-285 (Mar., 1984). Under the circumstances, SAW resonators undergo unwanted phenomenon such as frequency deviation or shift from the resonance frequency. On the other hand, in the case of the SAW filter designed for high power transmission, there often arise failures such as shortcircuit, wire breakage and the like due to the growth of hillocks, as is reported in "1983 IEEE Ultrasonic Symposium Proceedings", pp. 83-86 (1983). In particular, the second mentioned literature explains the generating mechanism of such failure as follows: "Strain in a substrate surface produced by the SAW induces an internal stress in the Al-thin film formed on the substrate surface, as a result of which Al-crystalline grain boundary is migrated in a region where the stress exceeds a threshold, giving rise to creation of the voids and hillocks. The migration of the grain boundary is believed to be ascribable to such mechanism as experienced in the thermal cycle of ICs, as reported in IEEE Trans, Parts, Hybrids and Packaging, Vol. PHP-7, pp. 134-138, Sep. 1971". The first to third literatures mentioned above disclose that the failures due to Al-migration can be dealt with by adding a small amount (1 to 4 wt. %) of copper (Cu) to Al forming the conductors in the semiconductor ICs and that this method is effective for suppressing the migration.
It is, however, noted that none of these first to third literatures mentioned above discloses any suggestion concerning formation of electrodes of the SAW device by sputtering. As the frequency becomes higher, the strains due to SAW become greater even when the power for transmission and the amplitude remain unchanged. Under the circumstances, in the case of the SAW resonator designed for handling signals of frequencies higher than 300 MHz such as a SAW filter employed in a transmitter of a cellular radio telephone system of a 800 MHz-band and the like, above all, an adequate useful life can not be assured because of occurrence of the above-mentioned migration in operation with high power and/or a large amplitude even with the Al-electrode added with Cu and formed through vacuum evaporation or EB evaporation disclosed in the first to third literatures enumerated above.
It is further noted that formation of Al-film containing an additive of Cu through the vacuum evaporation or EB evaporation tends to increase the hardness of the film, making it difficult to employ the wire bonding to another disadvantage. On the other hand, when a dry etching process is adopted for forming fine electrodes with a high precision for the purpose of enhancing the high frequency performance, the electrodes are susceptible to corrosion or the like injuries (e.g., due to the inclusion of Cl), giving rise to a problem that the yield is degraded significantly.
Another method of suppressing the electromigration is disclosed in "Semiconductor World" pp. 108-116, (July 1985) and others. According to this method, the crystal grain diameter of metal of which the deposited film is made is increased to suppress the electromigration. As specific means for increasing the crystal grain diameter in the thin film, a vacuum vapor evaporation method by resorting to the resistance heating as disclosed in the above-mentioned Transactions of The Institute of Electronics and Communication Engineers of Japan and a vacuum vapor evaporation method resorting to the use of an electron beam are adopted.
However, in the hitherto known vacuum vapor evaporation methods in which the resistance heating and the electron beam are employed, respectively, the concentration of an additive in each evaporation source becomes frequently non-uniform, the mean additive concentrations are different among the batches, and the vapor pressures and masses of individual elements are different from one another, which cause variations of the concentration or content of an additive in films among the individual lots or batches. As a consequence, it is impossible or at least very difficult to ensure an adequately long useful life for the SAW device provided with the Cu-added Al-electrodes formed through the vapor deposition based on the use of the vacuum evaporation or the electron beam evaporation, when the device is operated with high power (or a large amplitude) at high frequency even in case the power or amplitude remains unchanged, because of non-uniformity of the additive.